Journal bearings, bushings and other fabrications subject to loads bearing require a material that has a low coefficient of friction. As there is always relative motion between a bearing and its mating surface, friction is always a concern. Friction results in loss of power, generation of heat and increased surface wear. Therefore, a major consideration is that the material of a bearing, bushing and similar load-bearing items, has a low coefficient of friction.
As friction causes wear, also of concern is that the material be highly resistant to wear. Another major concern is that the material has sufficient strength or hardness for bearing loads that may continuously vary in magnitude.
Bearing alloys used in engine parts generally consist of either an aluminum or copper matrix containing alloying additions of an elemental metal which is not soluble in the matrix and exists therefore as a discrete phase. In aluminum matrix alloys the well-known and used elemental metals are tin, lead and cadmium and many alloys containing these constituents.
In the case of copper based alloys, however, only lead has been generally used. Lead is one of the few low melting point metals, which has the properties necessary to form a good bearing material with copper. The copper-lead alloys have given good service as engine bearing alloys but they have some limitations.
The two most serious problems with copper-lead alloys are:                (1) lead is a toxic substance and the use of lead in the production of alloys involves expensive control procedures; and        (2) the lead phase in copper-lead alloys is seriously affected by corrosive attack in hot engine oil. When engine oil is oxidized while hot (during the normal running of an engine) the oil breaks down to form peroxides and organic acids that dissolve the lead phase; this seriously weakens the bearing alloys and causes eventual malfunction and failure.        
Currently copper, bronze or brass lead-bearing alloys are cast using special techniques to achieve the fine distribution of lead necessary for machinability and tribological properties.
For certain applications, other metal and metal alloy materials have been used. For example, it is known to use tantalum, among other metals and alloys, with a solid lubricant exposed at its surfaces, for a roller bearing apparatus to be used in a molten metal bath.
Graphite and hexagonal boron nitride are both known to be solid lubricants useful in improving the machinability of metals and alloys, such as brass. Graphite and hexagonal boron nitride also have been used in metal-based, load-bearing articles. For example, the prior art describes a sliding member made by mixing a matrix material of iron-based powder containing chromium with 0.1 to 3.5 weight % hexagonal boron nitride and 0.1 to 3.5 wt. % graphite. The resultant powder mixture is compacted then sintered while in contact with copper or copper alloy such that the copper or copper alloy infiltrates into the iron-based matrix and the hexagonal boron nitride is distributed in the copper phase. The graphite reacts with the chromium to be precipitated as chromium carbide. The porosity of the obtained sintered product was at least 3% (by volume). However, neither graphite nor hexagonal boron nitride is known to be suitable as a full replacement for lead.
Shaped articles consisting of hexagonal boron nitride are known. Some prior art references describe shaped articles of hexagonal boron nitride having a density of at least 95% of its theoretical density consisting of pure hexagonal boron nitride that was hot isostatically pressed. However, the use of hexagonal boron nitride to form a highly densified metal-based material with low coefficients of friction and wear rates is not known.